Heart stimulator determining cardiac output, by measuring the systolic pressure, for controlling the stimulation

ABSTRACT

A heart stimulator has a circuit for determining cardiac output and for producing a control signal corresponding to the determined cardiac output, and a controller for controlling cardiac stimulation dependent on the control signal. The circuit for determining cardiac output includes a pressure sensor which measures pressure in the right ventricle and which generates an electrical pressure signal corresponding to the measured pressure, and an integrator supplied with the pressure signal which integrates the pressure signal between a start time and stop time to produce an integration result corresponding to the cardiac output, which is used to form the control signal. The pressure signal is bandpass filtered during a systolic phase to identify opening of a valve at the right ventricle as the start time, and to identify closing of the valve as the stop time.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a heart stimulator for electricalstimulation of the heart of the type having means for determiningcardiac output and control means for controlling the stimulation inresponse to the determined cardiac output, and wherein cardiac outputdetermining means includes means for measuring the pressure inside theright ventricle and producing a corresponding pressure signal, and meansfor producing from the pressure signal a control signal related to thecardiac output and supplying the control signal to the control means forcontrolling the delivered stimulation according to the control signal.

2. Description of the Prior Art

Several other manners of automatic adaption of pacemaker stimulationalgorithms and parameters are also known. A common shortcoming to allthese prior concepts is that no really appropriate criteria have beenfound for the algorithm and parameter optimization. Thus attempts havebeen made to e.g. optimize the AV-delay based on an assumed algorithmicrelation between cardiac rate or cardiac activity and an optimalAV-delay. However, the results have been unsatisfactory.

Because of the wide variety of conditions affecting the needs of thepatient, such as mental onset, nutrition, time of the day and season,diseases and individual peculiarities, it is not likely that analgorithmic relation will ever come close to the real situation. Asystem of this kind is unlikely to be effective unless the effectivenesscan be measured and used as feedback in a stimulation control. Todaypacemaker parameters are normally adjusted depending on diagnosis andthe experience of the medical personnel.

The above mentioned U.S. Pat. No. 5,156,147 describes a rate adaptivepacemaker having a variable rate cardiac stimulating pulse generator anda sensor for monitoring some physiologic parameter for adjusting thepulse generator stimulation rate to meet physiologic demands. Inaddition thereto a hemodynamic sensor is operative to provide an outputsignal representing the pumping performance of the heart in response tothe pacing stimulation. The hemodynamic monitoring sensor may measurethe right ventricular pressure, and it is mentioned that the hemodynamicparameter used for controlling the pacing output is determined from themeasured pressure signal.

The above subject is achieved in accordance with the principle of thepresent invention in a heart stimulator having a circuit for determiningcardiac output and for producing a control signal corresponding to thecardiac output, a controller for controlling cardiac stimulationdependent on the control signal, and wherein the cardiac outputdetermining circuit includes a pressure sensor which measures pressurein the right ventricle and which generates an electrical signal tocorresponding to the measured pressure, an integrator supplied with thepressure signal which integrates the pressure signal between a starttime and a stop time to produce an integration result, corresponding tothe cardiac output, for use as the control signal, and a bandpass filterconnected to the integrator and to the pressure sensor for filtering thepressure signal during a systolic phase to identify opening of a valveat the right ventricle as the aforementioned start time for theintegration and to identify a closing of this valve as theaforementioned stop time.

Also U.S. Pat. No. 5,183,051 discloses a method for continuouslydetermining cardiac output from measured arterial blood pressure data.Blood pressure is measured non-invasively or minimally invasively andthe stroke volume is determined from the stroke area under the pulsepressure curve between the start of the systolic phase and the dicroticnotch, said stroke area being corrected for any surface portions relatedto reflected pressure waves.

The methods of determining cardiac output according to the two lastmentioned publications are used for diagnostic purposes. Nothing ismentioned about using cardiac output as a parameter for controlling aheart stimulator.

The article K. Rohrbach, Ludwig-Maximilians-Universitat Munchen 1991,“Korrelation zwischen Schlagvolumen und abgeleiteten Druckparametern desrechten Herzens—Eine tierexperimentelle Studie-” discloses differentmethods of determining the stroke volume from a pressure signal obtainedin the right ventricle of a pig. According to one method in this thesisis the pressure signal integrated during systole.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a simple arrangementfor determining cardiac output as a measure of the effectiveness of thestimulation administered by a heart stimulator, and to use thedetermined cardiac output in a feedback procedure for adapting thestimulation as needed.

The above object is achieved in accordance with the principles of thepresent invention in a heart stimulator having a circuit for determiningcardiac output and for producing a control signal corresponding to thecardiac output, a controller for controlling cardiac stimulationdependent on the control signal, and wherein the cardiac outputdetermining circuit includes a pressure sensor which measures pressurein the right ventricle and which generates an electrical signalcorresponding to the measured pressure, an integrator supplied with thepressure signal which integrates the pressure signal between a starttime and a stop time tp produce an integration result, corresponding tothe cardiac output, for use as the control signal, and a bandpass filterconnected to the integrator and to the pressure sensor for filtering thepressure signal during a systolic phase to identify opening of a valveat the right ventricle as the aforementioned start time for theintegration, and to identify a closing of this valve as theaforementioned stop time.

With the heart stimulator according to the invention it is possible in asimple and reliable way to adjust algorithms and one or more stimulationparameters dynamically in response to environmental and demand changesto maintain an optimum cardiac output. Examples of parameters which canbe adjusted in such a feedback procedure are AV-delay, stimulation rate,refractory period, stimulation pulse energy, duration, and amplitude.

In an embodiment of the heart stimulator according to the inventioncardiac output determining circuit measures the cardiac output on a beatto beat basis. It is then possible to calculate energy effectiveness perbeat and hence energy consumption over time for a certain cardiacoutput. This is of vital importance in a heart stimulator since it isessential not to stress the heart and not to use more energy than neededin a certain situation.

According to another embodiment of the heart stimulator of the inventiona sample and hold circuit is connected to the pressure sensor to holdthe measured pressure value at the ventricle outflow valve opening, theintegrator being controlled by the sample and hold circuit to continueintegration as long as the measured systolic pressure is higher than theheld valve opening pressure value. This is a practical way of definingthe limits for the integration. The determination of these limits arenot too critical, since the integrated area has the character of arelative measure and not an absolute one. Thus in practice a change ofan input parameter is performed and the corresponding change of theintegrated area, representing cardiac output, is observed, and with theaid of this information the next change of the input parameter in orderto reach an optimum cardiac output is determined.

In another embodiment of the heart stimulator of the invention, theintegrator is adapted to determine, as a measure of cardiac output, themagnitude of an area in the pressure versus time plane limited by themeasured ventricular pressure curve as a function of time from ventricleoutflow valve opening to valve closure and a straight line between themeasured pressure values at ventricle outflow valve opening and closure.The pressure difference between the right ventricle and the pulmonaryartery is the driving force accelerating blood out of the ventriclethrough the opened valve and the speed of blood flow is determined bythis pressure difference and by the blood's density. Thus, from atheoretical point of view, the ventricle over pulmonary artery pressuredifference is the relevant quantity to be integrated for determining thevolume output per beat or cardiac output. During the ejection phase,however, the exact value of the pulmonary artery pressure cannot bemeasured from inside the ventricle. The pressure at the beginning of theejection phase and at the end thereof can, however, be measured and thepulmonary artery constitutes an elastic system and the pressure isincreasing approximately linearly with the enclosed volume. Consequentlya straight line between the pressure at valve opening and the pressureat valve closure is a good approximate average value of the aorticpressure built up.

In a further embodiment of the heart stimulator of the invention theintegrator is adapted to determine, as a measure of cardiac output, themagnitude of the area in the pressure versus time plane below themeasured ventricular pressure curve and above a selected constantthreshold level below the measured ventricular pressure, the integratorbeing controlled to start the integration as soon as the measuredventricular pressure exceeds said threshold level and stop theintegration when the measured pressure drops below the threshold level.This is a simplified embodiment of the heart stimulator according to theinvention with reduced complexity of electric circuitry, memory meansand used algorithms, in which yet normally a good estimate of thecardiac output is obtained for controlling purposes. Since the pressureis steeply rising prior to valve opening and steeply falling after valveclosure, using a constant level for determining start and end points ofthe integration will not significantly shift these points from actualvalve opening and closure points, and provided that the threshold levelis not changed relative to the pressure at valve opening from one beatto the next one, the difference between the measured cardiac output andactual cardiac output will be approximately constant. This differencebetween measured and actual cardiac output values can then be eliminatedby forming the difference between consecutive measured values.

DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a longitudinal view and FIG. 1B shows an end view of theend portion of a ventricular lead suitable for use with the heartstimulator according to the invention.

FIG. 2 is a principal diagram of an input circuitry for stimulation andQRS-detection of the heart stimulator according to the invention.

FIG. 3 shows pressures measured in different parts of the heart and theaortic and pulmonary artery pressures as a function of time as well asphonocardio- and electrocardiograms for illustrating one method ofdetermining cardiac output in the stimulator according to the invention.

FIGS. 4-6 show the same pressure curves and phono- andelectrocardiograms for illustrating alternative methods of determiningthe cardiac output in the heart stimulator according to the invention.

FIG. 7 shows a first embodiment of a cardiac output measuring circuitryof the heart stimulator according to the invention.

FIG. 8 shows a second embodiment of the circuitry for determiningcardiac output from the pressure signal in the heart stimulatoraccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the heart stimulator according to the invention cardiac output isdetermined as the time integral of the pressure in the ventricle duringthe systolic phase of the heart. The pressure is preferably measured bya pressure sensor 2 positioned behind the tip of a ventricular electrodelead intended to be used with a heart stimulator according to theinvention, see FIGS. 1A and 1B. The lead tip 4 is electrically connectedto the heart stimulator for conducting stimulation impulses to hearttissue and, in most applications, also for sensing electrical activityarising from heart contraction. The pressure sensor 2 is preferably apiezoelectric sensor also connected to the heart stimulator fordelivering a pressure signal representing the sensed pressure.

FIG. 2 shows the input circuitry of a heart stimulator according to theinvention. A stimulation capacitor C is charged to a predeterminedstimulation voltage V_(stim) from a battery in the heart stimulator (notshown in the figure), the switch S then being in the position shown inFIG. 2. A stimulation pulse can thereafter be delivered from thecapacitor C through the electrode lead to the tip 4 by moving the switchS to its second position for connecting the capacitor C to the lead.

As mentioned above the lead tip 4 is also used for sensing electricalheart activity and the sensed signals are supplied to a QRS-detector 6for QRS-detection for use in the control of the pacemaker.

The following description will be made with respect to the leftventricle. However, as the vascular system is a closed system the bloodvolume flowing in different parts of the system will be essentially thesame and therefore the description is also applicable to the rightventricle. In steady state left and right ventricular cardiac outputwill be equal. Temporary variations will result in a redistribution ofpressures and a new steady state situation will rapidly emerge.

FIG. 3 shows measured pressure curves during a cardiac cycle. Duringsystole the pressure inside the ventricle is rapidly increasing. At acertain level 8 the pressure exceeds the pressure in the aorta and theaortic valve opens. This is the start of the ejection phase in whichblood is driven out of the ventricle into the aorta. The pressuredifference between the ventricle and the aorta is the driving forceaccelerating blood out of the ventricle through the open valve. Thespeed of the blood flow is determined by this pressure difference andthe density of the blood. The area of the valve opening is substantiallyconstant from beat to beat and the volume output per beat or cardiacoutput is found to be proportional to the time integral of the pressuredifference over the ejection time from valve opening 8 to valve closure10 in FIG. 3.

From a theoretical point of view the difference between the pressure inthe ventricle and the aortic pressure is thus the pertinent quantity tobe integrated. During the ejection phase, however, the exact value ofthe aortic pressure cannot be measured from the inside of the ventricle,but only at the start 8 and at the end 10 of the systolic phase. Theaorta constitutes an elastic system and the pressure is increasingapproximately linearly with the enclosed volume. A straight line betweenthe valve opening 8 and the valve closure 10 therefore forms a goodapproximation of the build-up of the aortic pressure. This has beenverified by comparative measurements of cardiac output by a flowmeterand by the measurements described above.

Thus, in one embodiment of the heart stimulator according to theinvention, cardiac output is determined by integration of the surfacedelimited by the pressure curve 12 and the assumed linearly varyingaortic pressure 14.

To simplify circuitries, memories, and algorithms needed for determiningcardiac output as described above modifications of the describedprocedure are possible.

As noted above the pressure rise and fall prior to valve opening andafter valve closure respectively are steep. During this rise and fallthe elastic effect from the aorta has no leveling or smoothing influenceon the variation of the pressure, and because of the steepness of thepressure curve a constant level, represented by the straight line 16 inFIG. 4, can be used for defining the start and end points 18, 20 of theintegration. Using the intersection points between this straight line 16and the pressure curve 12 as start and end points for the integrationinstead of the intersection points between e.g. the inclined line 14 andthe pressure curve 12 as in FIG. 3 will only result in minor shifts ofthe start and end points. If the level of the line 16 relative to thepressure at the valve opening is the same from one beat to the next one,the difference between the actual cardiac output and the measuredcardiac output will be approximately constant and have practically noinfluence on a relative measurement of cardiac output which issufficient for the control of the heart stimulator according to theinvention.

As such a reference level for the integration half the peak systolicpressure from a base line can be chosen.

In another alternative embodiment of the heart stimulator according tothe invention the pressure inside the ventricle is monitored by a sampleand hold circuit. The pressure measured at the point of valve opening isthen Hold and the integration is started and continued as long as thesystolic pressure remains higher than the hold value or till valveclosure. These two possibilities are illustrated in FIGS. 5 and 6respectively. Thus a horizontal straight line 22 through the pressure atvalve opening 24 forms the reference for the integration. In theembodiment illustrated in FIG. 5 the integration is stopped at theintersection point 26 between the falling flank of the pressure curve 12and the reference line 22 and in the embodiment illustrated by FIG. 6the integration is stopped at valve closure 10 as also illustrated inFIG. 3.

The embodiment incorporating sample and hold circuits will be describedin greater detail below with reference to FIGS. 7 and 8.

In FIGS. 3-6 the discussed pressure curve 12, which continues as a curveVK in the second half of the shown cardiac cycle, represents thepressure measured in the left ventricle. The curve Ao shows the aorticpressure and the dotted curve VF illustrates measured pressure in theleft atrium.

FKG is a phono cardiogram which shows increased noise at valve openingand valve closure. This increased noise can be detected by an acousticsensor and used for detecting valve opening and closure.

The pressure curve HK represents the pressure in the right ventricle andthis curve can be used for determining cardiac output in an analogousway as the pressure curve 12. The curve AP represents measured pressurein the pulmonary artery and the dotted curve HF the pressure in theright atrium. EKG denotes an electrocardiogram.

By a pressure sensor with a reasonably high frequency responsepositioned in the ventricle it is possible to detect pressure artifactsresulting from the acceleration and deceleration of fluid at valveopening and closing. These pressure artifacts appear as damped periodicoscillations, the frequency of which is determined by the elasticproperty of the system formed by e.g. aorta, or pulmonary artery, andventricle, and the mass density of the fluid. By using a narrow bandpassfilter valve opening and closure can be detected from these oscillationsin the pressure signal.

A first embodiment of the circuitry for determining cardiac output inthe heart stimulator according to the invention is shown in FIG. 7. Thiscircuit includes two sample and hold circuits 28, 30, and an integrator32.

At valve opening the integration is started and the first sample andhold circuit 28 is brought into Hold position by the switch 34. In thisway the end diastolic pressured is stored on the hold capacitor C₁ ofthis circuit.

At valve closure the integration is stopped and the second sample andhold circuit 30 is brought into Hold position with the aid of the switch36. The end systolic pressure is then stored on the hold capacitor C₂ ofthis circuit.

Start and stop of the integration is controlled by the switch 38.

In the embodiment shown in FIG. 7 the end diastolic pressure is suppliedto the integrator 32 as integral reference.

By a multiplexor 40 the diastolic and systolic pressures as well asintegrated pressure values are supplied to an AD-converter 42 fordigitization and transfer to the heart stimulator controller.

To make the integrator 32 ready for the next measurement it is reset byclosure of the switch 44.

In this embodiment cardiac output thus will be calculated according tothe formula

Cardiac Output=Pressure Integral−(End Systolic Pressure−End DiastolicPressure)×time/2

as explained in connection with FIG. 3.

FIG. 8 shows an alternative embodiment of the circuitry in which thesecond sample and hold circuit is replaced by a comparator circuit 46.By the first sample and hold circuit 28 the end diastolic pressure isstored on the capacitor C₁. The stored end diastolic pressure is used asan integral reference and is supplied to the comparator 46 and theintegrator 32. In the comparator 46 the sensed pressure signal,amplified in the pressure amplifier 48 is compared with the reference.As the pressure signal exceeds the integral reference value thecomparator 46 delivers an output signal closing the switch 38 forstarting the integration of the pressure signal. The integration isstopped when the pressure signal drops below the integral reference,i.e. the stored end diastolic pressure.

The integral reference value is also fed to the integrator 32 such thatthe area delimited by the sensed pressure curve and the end diastolicpressure in the pressure versus time plane is determined by theintegrator 32.

Similarly as in the embodiment in FIG. 3 an integrator reset switch 44is provided and a multiplexor 40 for supplying the end diastolicpressure and the integration value to the AD-converter 42 fordigitization and transfer to the pacemaker controller.

Valve opening and closing can be detected by filtering the pressuresignal as described above and the detected opening and closing eventsare controlling the switches of the circuitry in FIGS. 7 and 8. Asmentioned above a pressure sensor, preferably of a piezoelectric type,is used. However, any other type of pressure sensor having asufficiently high frequency range response, typically 200-500 Hz, can beused.

Alternatively heart sound from the valves can be used for determiningvalve opening and closure. As appears from the phono cardiograms inFIGS. 3-6 there is a characteristic increase in the recorded sounds atvalve opening and closure.

As still another alternative valve opening and closure can be determinedfrom recorded electrocardiograms. The electrocardiograms can be recordedby the ordinary electrode lead used for stimulation or by a separatelead.

Yet another alternative for determining valve opening, and consequentlythe starting point for the integration, in case of pacemakerstimulation, consists in adding an appropriate delay, typically 40 msec,from the timing position for a delivered stimulation pulse.

Cardiac output can also be determined from a measured AC-impedancesignal. The AC-impedance signal is varying with time in a similar way asthe pressure signal. The volume of blood contained in the heart isreflected in the value of the AC-impedance. More blood contained in theheart results in a lower impedance. For using the impedance as a measureof the pressure, it is necessary that the impedance is measured locallyin the ventricle, i.e. by a bipolar electrode.

For determining the impedance as a measure of the pressure a separatemeasurement catheter can be used, as well.

With the heart stimulator according to the invention the possibilitiesfor satisfying the patient's need with a minimum energy consumption areimproved.

Energy consumption per heartbeat is fairly constant if other conditionsfor the patient remain essentially unchanged. Factors affecting energyconsumption are hypertension and heart onset caused by the autonomousnerve system and hormone release. Both these factors increase energyconsumption per heartbeat. For a patient suffering from hypertension theheart has to pump blood against an increased pressure when opening theoutlet aortic or pulmonary valve. Heart onset is related to themetabolic demand, e.g. due to workload.

Metabolic demand is setting the level of the peripheral vascularresistance which can preferably be determined from the end diastolicpressure, i.e. the aortic pressure at aortic valve opening. As theperipheral vascular resistance increases the pressure drops more rapidlyand a lower end diastolic pressure results. Peripheral vascularresistance can alternatively be determined from venous blood return tothe heart or indirectly by means of sensors. Examples of such sensorsare body activity sensors, based on body movements, or muscular soundwaves or metabolic sensors sensing oxygen or carbon dioxide saturation,pH value or blood temperature. Utilizing a pressure sensor is, however,to prefer, since combined monitoring of different physiologicalparameters is then possible by one single sensor. Thus metabolic needcan be determined from the end diastolic pressure, heart inability canbe detected by sensed ischemic episodes, the heart can be monitored bysensed normal spontaneous systolic, stimulation capture can be monitoredby sensed evoked systolic response and cardiac output can be monitoredby determining the systolic pressure integral, as described above. Sucha pressure sensing system will closely mimic the natural system of thebody.

From a signal representing the metabolic demand measured according toone of the techniques mentioned above, with possible compensation forpathological factors, such as ischemia or hypertension, a desiredcardiac output control value can be determined by a suitable algorithmor from tables. The actual cardiac output is determined from thesystolic pressure integral as described above and stimulation parametersof the heart stimulator according to the invention are changed in smallsteps followed by evaluation of the resulting cardiac output after eachchange. If cardiac output is increased the new parameter value willreplace the old one, and if cardiac output decreases, no parametricchange is performed.

As an example of this way of controlling the heart stimulator a singleparameter adjusting system in the form of a rate adaption system will bedescribed.

With a patient at rest the stimulation rate is decreased in small stepstill a heart rate is reached which results in the desired cardiacoutput. If the workload is increased the desired cardiac output isincreased, too. The stimulation rate is increased in steps as long asthe resulting cardiac output is increased. At a certain rate limitvenous blood return will not be sufficient for refilling the heartbetween consecutive systoles, and further rate increase will thenactually decrease cardiac output. At this point the heart stimulatoraccording to the invention will automatically stop further rate increaseand keep the stimulation rate at the value which gives a maximum cardiacoutput.

The adjustment of the stimulation can also include changing stimulationparameters other than the stimulation rate, like stimulation pulseamplitude, duration and shape of the pulse. At the site of an implantedstimulation lead different conduction patterns can arise for differentstimulation energy levels. Thus e.g. a conduction barrier can exist forenergies below a certain threshold. In such a case the propagatingdepolarization wave must find another way around the area in questionwith an associated delay in the conduction pattern. Such a delay willalso affect the time progress of the contraction pattern and reducecardiac output. This phenomenon occurs more easily if the implanted leaddoes not have an optimal location. A sufficiently high stimulationenergy will stimulate a larger tissue volume around the lead and can inthis way “compensate” for the less suitable positioning of the lead.

Stimulating leads are implanted into the right ventricle. From a site inthe right ventricle a minimum stimulation energy is needed to overcomedelay across the septum and to match left and right ventriclecontraction patterns. For a patient having left bundle branch block itcan be difficult to reach the left ventricle from the right ventricle,anyhow a higher energy can be required.

In the heart stimulator according to the invention a single parameter,or a combination of parameters, can be optimized, each parameter thenbeing adjusted and evaluated in a sequence of optimization. Thus, e.g.the AV-delay in a two- chamber system can be varied to reach an optimumcardiac output per beat and the heart rate is accordingly decreased tomaintain the desired cardiac output.

The time delay after a delivered stimulation pulse can be in the rangeof 0-100 msec.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventor to embody within the patentwarranted hereon all changes and modifications as reasonably andproperly come within the scope of his contribution to the art.

I claim:
 1. A heart stimulator comprising: a circuit for determiningcardiac output and for producing a control signal corresponding to saidcardiac output; a controller for controlling cardiac stimulationdependent on said control signal; and said circuit including a pressuresensor adapted to sense pressure in a right ventricle and to generate anelectrical pressure signal corresponding to the sensed pressure, anintegrator supplied with said pressure signal which integrates saidpressure signal between a start time and a stop time to produce anintegration result, corresponding to said cardiac output, forming saidcontrol signal, and wherein said pressure signal is bandpass filteredduring a systolic phase to identify opening of a valve at said rightventricle as said start time and closing of said valve as said stoptime.
 2. A heart stimulator as claimed in claim 1 wherein said circuitdetermines said cardiac output on a beat-to-beat basis.
 3. A heartstimulator as claimed in claim 1 wherein said circuit determines saidcardiac output as an average value over a plurality of heartbeats.
 4. Aheart stimulator as claimed in claim 1 further comprising a lead havingan electrode tip for conducting stimulation pulses to heart tissue andfor sensing electrical heart activity, said lead also carrying saidpressure sensor.
 5. A heart stimulator as claimed in claim 4 whereinsaid pressure sensor comprises a piezoelectric pressure sensor disposedbehind said electrode tip.
 6. A heart stimulator as claimed in claim 1wherein said integrator determines, as a measure of said cardiac output,a magnitude in an area of a pressure versus time plane limited by acurve represented by said pressure signal as a function of time betweensaid start time and said stop time, and a straight line proceedingbetween respective values of said pressure signal at said start time andsaid stop time.
 7. A heart stimulator as claimed in claim 1 furthercomprising a sensor for determining a metabolic need of a patient andfor generating a metabolic need signal corresponding said metabolicneed, and a comparator for comparing said metabolic need signal withsaid integration result, to obtain a comparison result, said comparisonresult forming said control signal.
 8. A heart stimulator as claimed inclaim 7 wherein said metabolic need sensor comprises a flow sensoradapted to measure venous blood return to the heart.
 9. A heartstimulator as claimed in claim 7 wherein said metabolic need sensorcomprises a further pressure sensor adapted to detect end diastolicpressure.
 10. A heart stimulator as claimed in claim 7 wherein saidmetabolic need sensor comprises an activity sensor.
 11. A heartstimulator as claimed in claim 7 wherein said metabolic need sensorcomprises a sensor adapted to measure a blood parameter selected fromthe group consisting of blood oxygen saturation, blood carbon dioxidesaturation, blood pH and blood temperature.
 12. A heart stimulator asclaimed in claim 1 comprising a stimulation pulse emitter foradministering said stimulation, said stimulation pulse emitter having astimulation rate, a stimulation pulse amplitude, a pulse duration and apulse shape associated therewith, and wherein said control signaloperates said stimulation pulse generator to adjust at least one of saidparameters.